This invention is related to sonic signal transducers, including ultrasonic transducers. The invention has particular use as a transducer in an ultrasonic vortex velocity sensing device.
It is well known in the art to use sonic signals in various measuring and testing methods and apparatus. One such use was set forth in U.S. Pat. application, Ser. No. 857,328, filed Sept. 12, 1969, now U.S. Pat. No. 3,680,375 and entitled "Sonic Velocity Sensing." This patent discloses a method and apparatus for sensing the rate of Karman vortex generation. In accordance with the well known Karman vortex phenomena, a fluctuating flow field, more commonly referred to as a Karman vortex street or trail, is formed in the wake of a strut mounted in a fluid stream. The fluid may be either gaseous or liquid. The rate of vortex generation provides an indication of the relative velocity between the body to which the strut is attached and the movement of the fluid stream. As set forth in the above patent application, a sonic signal is transmitted across the fluid stream and is modulated by the vortices in the stream. An indication of velocity is provided by detecting the modulation frequency of the sonic signal.
In use of the above vortex velocity sensor, it has been found that sonic signals reflected from a receiving transducer and then back again from a transmitting transducer will, when out of phase with a transmitted signal, cause destructive addition which reduces the modulation content of a received signal. Under zero flow conditions, the frequency of the transmitted signal can be tuned such that the reflected signal will be in phase with the transmitted signal. However, under flow conditions where the sonic energy is swept downstream at the fluid velocity, the reflections may take a variety of path lengths, depending on the flow conditions. Proper tuning of the transmitted frequency will also depend on temperatures which affect the speed of sound. Under given flow conditions, a frequency can be selected which gives reasonable test results. However, this critical frequency must generally be maintained within approximately 0.25% accuracy.
A number of techniques are employed to avoid the necessity of maintaining a critical frequency in transmitting the sonic signal. One such technique is the use of an acoustical lens having a conically shaped outer surface attached to the acoustic transmitting transducer. The conical surface tends to disperse any sonic reflections such that they do not interfere with the transmitted signals. The technique introduces additional problems in that bonding between the acoustic lens and the transducer is extremely critical. Also it is required that the resonant frequency of the acoustic lens is matched very closely to the resonant frequency of the transducer. These limitations cause considerable manufacturing problems and require that each device be individually tested to achieve proper operation.
Another technique employed is to mount the transmitting transducer without an acoustic lens, but at an angle such that reflections from the face of the transmitting transducer misses the receiving transducer on subsequent bounces. This technique causes the loss of considerable acoustic energy and also presents complications in the manufacture of the instrument since it is required that transducer holders be inclined with respect to each other.
It is therefore an object of this invention to provide a sonic transducer which substantially reduces the problems caused by interference between reflected and transmitted sonic signals.
It is a further object of this invention to provide such a sonic transducer which is relatively free from manufacturing and economic complications presented by other prior art systems.